Large plants are often more conspicuous and more attractive for associated animals than small plants, e.g. due to their wider range of resources. Therefore, plant size can positively affect species richness of associated animals, as shown for single groups of herbivores, but studies usually consider intraspecific size differences of plants in unstandardised environments. As comprehensive tests of interspecific plant size differences under standardised conditions are missing so far, we investigated effects of plant size on species richness of all associated arthropods using a common garden experiment with 21 Brassicaceae species covering a broad interspecific plant size gradient from 10 to 130 cm height. We recorded plant associated ecto- and endophagous herbivores, their natural enemies and pollinators on and in each aboveground plant organ, i.e. flowers, fruits, leaves and stems. Plant size (measured as height from the ground), the number of different plant organ entities and their biomass were assessed. Increasing plant size led to increased species richness of associated herbivores, natural enemies and pollinating insects. This pattern was found for ectophagous and endophagous herbivores, their natural enemies, as well as for herbivores associated with leaves and fruits and their natural enemies, independently of the additional positive effects of resource availability (i.e. organ biomass or number of entities and, regarding natural enemies, herbivore species richness). We found a lower R2 for pollinators compared to herbivores and natural enemies, probably caused by the high importance of flower characteristics for pollinator species richness besides plant size. Overall, the increase in plant height from 10 to 130 cm led to a 2.7-fold increase in predicted total arthropod species richness. In conclusion, plant size is a comprehensive driver of species richness of the plant associated arthropods, including pollinators, herbivores and their natural enemies, whether they are endophagous or ectophagous or associated with leaves or fruits.

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Abstract. Plant size has been hypothesized to be a major driver of biotic interactions. However, it is little understood how plant size affects plant mutualists vs. antagonists and the plant's resulting reproductive success. We established a common garden experiment covering an interspecific plant size gradient (from 10 to 130 cm height) across 21 annual Brassicaceae species, thereby standardizing features of habitat and surrounding landscape. We assessed flower-visiting pollinators and florivores (pollen beetle adults and larvae) and the resulting effects of all these flower-visiting insects on plant reproductive success. Besides flower characteristics (size, abundance, color), plant size had a generally positive effect on abundance and species richness of pollinators as well as on abundance of pollen beetle adults and larvae. Pollen beetles reduced seed number as well as thousand-seed weight, whereas pollinators increased seed number only. Overall, increasing plant size led to less thousand-seed weight but had no effect on seed number, indicating counterbalancing effects of herbivory and pollination. In conclusion, seed number of large plant species should benefit from locations with many pollinators and few herbivores and small plant species' seed number from locations with few pollinators and many herbivores.

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